schieffer: good evening. scott s off tonight, i m bob schieffer. well, it s happened again two american military men were killed in afghanistan today by an afghan policeman. it happened in a village in afghanistan s far west where u.s. troops have been teaching the afghans to take over their own security. this is just the latest attack by afghans who are supposed to be our allies. they took the lives of 12 americans in 2010. last year that number doubled and already this year there have been 25 who have been killed. david martin has more on today s shooting and this troubling trend. reporter: it happened at a graduation ceremony like this. villagers trained by americans to serve as a local police force. a brand new policeman stepped forward to receive his weapon and immediately turned it on his trainers killing two members of u.s. special forces. it s a kick in the gut of the u.s. strategy of training afghans to take over the fighting so americans can come home. for fami